Dandruff is typically not a serious problem. With cats it can sometimes be an indication that they're not getting enough fat in their diet, which can occur with many generic or low quality foods. You can try switching to a better quality food or even adding a fish oil supplement to your cat's diet to see if the dandruff improves. If the dandruff includes scabs, crusting or itching, or if your cat is not eating or appears particularly lethargic, this can be a sign of a more serious problem and you should take him to your veterinarian.